In many gas turbine engines, the fuel system for regulating the flow of fuel to the combustion chamber consists of one or more fuel nozzles arranged in the combustion chamber, a fuel pump for pressurizing fuel from the fuel supply, a fuel metering unit for controlling the flow of fuel to the fuel nozzles and one or more fuel manifolds fluidically connecting the fuel metering unit to the fuel nozzles.
During engine start-up, fuel is pumped from the fuel supply to the fuel metering unit by the fuel pump and, once a sufficient start-up pressure is attained, the pressurizing valve of the fuel metering unit opens and fuel is supplied to the fuel nozzles via the fuel manifold. Thereafter, the metering valve of the fuel metering unit modulates the rate of fuel flow from the fuel supply to the nozzles. As such, a single, continuous flow path exists from the fuel metering unit, through the fuel manifold, to the fuel nozzles.
In more advanced gas turbine aircraft engines, however, the fuel system includes additional components and has multiple flow paths. For example, a dual flow path fuel system may include multiple sets of fuel nozzles (i.e., a primary fuel nozzle and a secondary fuel nozzle), two fuel manifolds (i.e., a primary manifold and a secondary manifold), and a flow divider valve arranged downstream of the fuel metering unit. In such systems, the flow divider valve splits the flow of fuel from the fuel metering unit into two distinct flow paths, namely a primary flow path and a secondary flow path.
In dual flow path fuel systems, fuel is delivered to the primary and secondary nozzles in a predetermined and scheduled manner. For instance, during engine start-up, fuel is initially supplied only to the primary fuel nozzles. However, once the fuel from the primary fuel nozzles is burning in a steady and satisfactory manner, fuel is thereafter supplied to the secondary nozzles. Put another way, the primary flow path provides a pilot flow, or a flow which initiates the combustion process, while the secondary flow path provides a main flow, or a flow which supplements and intensifies the combustion process once the pilot flow is burning steadily.
Fuel systems for some gas turbine engines require an ecology function that removes a set amount of fuel from the fuel nozzles and manifolds upon cessation of engine operation. The removal of fuel serves two purposes. It prevents the fuel from trickling into the still hot combustion chamber, which causes the fuel nozzles in the engine to coke and/or the engine to smoke. This hinders engine performance and leads to premature failure of the nozzle. The removal of fuel also keeps the fuel from vaporizing into the atmosphere, which is not acceptable from an environmental standpoint.
Prior fuel systems such as disclosed in U.S. Pat. No. 5,809,771 to Wernberg use one ecology valve and one flow divider valve for all the nozzles when the fuel manifolds are small in diameter and there are relatively few nozzles. However, using one flow divider valve to split flow between multiple nozzle assemblies results in the addition of a second flow manifold. For small engines this is only a small weight and cost penalty. Larger engines utilizing many nozzle assemblies require proportionately larger and heavier ecology and flow divider valves as well as an additional large and heavy fuel manifold. To avoid this additional manifold some larger engines have small flow divider valves at each nozzle assembly. However, these flow divider valves do not provide the ecology function.